J. Eur. Opt. Society-Rapid Publ. 22, 29( 2026) 297
Table 1. Peak fitting results of the obtained XRD profile of electroplated Cu surface.
( hkl) |
2hv(°) |
d( Å) |
Height |
Area |
Area(%) |
FWHM(°) |
Crystalline size( nm) |
( 1 1 1) |
43.279 |
2.0888 |
1156 |
15563 |
100.0 |
0.468 |
47 |
( 2 0 0) |
50.831 |
1.8098 |
370 |
6875 |
44.2 |
0.646 |
23 |
( 2 2 0) |
74.076 |
1.2788 |
210 |
4250 |
27.3 |
0.695 |
23 |
( 3 1 1) |
89.877 |
1.0905 |
207 |
4771 |
30.7 |
0.822 |
20 |
( 2 2 2) |
95.099 |
1.0440 |
71 |
1208 |
7.8 |
0.703 |
26 |
The difference between the EBSD and XRD results arises from both their measurement principles and the nature of what they detect. EBSD identifies grains on the basis of orientation differences and merges slightly misoriented crystallites into a single grain if the misorientation is below the threshold angle. In contrast, XRD estimates the size of crystallites, i. e., coherently diffracting domains within a grain that correspond to subdomains of a crystal. Consequently, the crystallite sizes obtained by XRD analysis tend to be smaller, whereas the grain sizes measured by EBSD analysis appear larger. In any case, the electroplated Cu surface exhibits fine crystalline microstructures; thus, the sand-like texture observed on the machined electroplated Cu surface, as shown in Figure 2b, is considered to result from these fine microstructures. Moreover, a possible reason for the absence of the step structures on the machined electroplated Cu surface, as shown in Figure 1b, is that its crystal grains are too small to significantly affect the machining process.
4 Conclusions
We investigated the roughness and microstructures of the electroplated Cu machined by ultraprecision diamond turning, as well as those of the OFC and electroless-plated NiP for comparison.
The average RMS surface roughnesses measured over a large area of 140 lm 105 lm using a WLI in ascending order were 0.83 nm for electroplated Cu, 1.26 nm for electroless-plated NiP, and 1.65 nm for OFC. Step structures were observed on the OFC surface, probably caused by crystal grains, leading to larger RMS roughnesses. Concentric waves were observed on the NiP plating; however, they are considered to be caused not by the material itself but by the motion error of the ultraprecision turning machine. The electroplated Cu surface appeared the smoothest with no step structures.
For a smaller area of 1 lm 1 lm measured using an AFM, the average RMS surface roughnesses in ascending order were 1.44 nm for electroless-plated NiP, 1.74 nm for electroplated Cu, and 2.29 nm for OFC. The OFC surface showed tiny holes, whereas the electroplated Cu surface showed a sand-textured finish. In comparison, the electroless-plated NiP surface was smoother than the other two.
Focusing on the electroplated Cu, the results of PSD analysis can be summarized as follows. At spatial frequencies below 2 10 4 mm �1, the PSD of the electroplated
Cu surface is lower than that of the OFC surface. When the range is narrowed to spatial frequencies below 1 10 3 mm �1, the PSD of the electroplated Cu surface becomes comparable to that of the electroless-plated NiP surface and remains lower than that of the OFC surface.
Microstructural analysis by EBSD revealed fine crystal grains on the electroplated Cu surface ranging from 40 to 280 nm, whereas XRD indicated smaller crystallite sizes of 20 – 47 nm, confirming that the electroplated Cu is fully crystallized and polycrystalline. The discrepancy between the EBSD and XRD results arises from differences in their measurement principles. In any case, the electroplated Cu was found to be composed of microcrystalline grains, as confirmed by both EBSD and XRD analyses. Therefore, the sand-like texture observed on the machined electroplated Cu surface is likely due to the presence of these microcrystalline grains. Moreover, a possible reason for the absence of the step structures on the machined electroplated Cu surface is that its crystal grains are too small to significantly affect the machining process.
Overall, the findings demonstrate that electroplated Cu provides superior surface smoothness and machinability compared with OFC, although it is slightly inferior to electroless-plated NiP at high spatial frequencies, highlighting its suitability for the ultraprecision machining of optical components, where both high shape accuracy and small surface roughness are essential.
Acknowledgments
We thank Hideyuki Aizawa of Sendai Nikon Corporation for turning the specimens. We also thank Hideki Munakata of Chiba Institute of Technology for measuring the surface topographies.
Funding This research received no external funding.
Conflicts of interest The authors declare that they have no conflicts of interest.
Data availability statement
The data that support the findings of this study, including those not shown in this article, are available from the corresponding author upon reasonable request.
Author contribution statement
HT conceptualized and supervised the study, participated in all experiments, interpreted all the data, and wrote the manuscript.